The cooperative clinical study evaluates the effect of nutritional therapy in the progression of chronic renal disease. The specific goals of the clinical trial are as follows: 1) To compare the effect of protein restricted diets on the progression of chronic renal disease; 2) determine patient acceptance of these diets; and 3) evaluate their nutritional safety and other effects. Restriction of protein and phosphorus will be the primary intervention, and the rate of glomerular filtration will be the primary outcome measure. The study diets to be tested are as follows: 1. Diet M (moderate protein intake) - 1.0 to 1.4 gm protein and 16 to 20 mg phosphorus per kg standard body weight per day. 2. Diet L (low protein intake) - 0.55 to 0.60 gm protein and 5 to 10 mg phosphorus per kg standard body weight per day. 3. Diet K (very low protein intake) - 0.28 gm protein and 4 to 9 mg phosphorus per kg standard body weight per day, supplemented by a mixture of essential amino acids and ketoacid analogs. 4. Diet J (very low protein intake) - same protein intake as Diet K, but supplemented with essential amino acids. Patients will be randomly assigned to Diet M, Diet L or Diet K. A daily vitamin and mineral supplement will be administered to all study patients. Patients on the very low protein diets (Diets K and Diet J) will receive low protein foods as an aid to enhance adequate nutritional intake as well as compliance to the treatment regimen. The latter is in addition to the mixture of amino and ketoacids and essential amino acid supplementation.